I don't recall coming across anything in the base book that dealt with how fast credits should be acquired. I realize that this system is more flexible and is encouraging different groups to play their own style however some GM's may be at a loss without some rough guidelines so I would suggest including one line in the book maybe bundled in with acquiring xp about a normal, slow, and fast rate of acquiring credits.
In our games it has ussually been around 2500 a player (from various sources, bounties, selling items, rewards) with occasionally a little more if they go out of their way to earn it. That may seem like a lot but when you have ship repairs, and fuel to pay for along with saving for new weapons and gear I think it has been pretty reasonable.
I'd have to agree; credits in of themselves are a great motivator (see Han's money problems from Episode IV to the begining of Episode VI). I've played in dozens of RPGs where they said that money wasn't suppose to be important, but it kept becoming a hassle to deal with. We (the players) never had enough money to buy anything significant, but our enemies (NPCs of the GM) always had the means to acquire anything relevant to the plot that week.
Having credits lets players save for something that they really want. If you have a Force Exile with no idea how to craft a lightsaber, he might just very well start saving up every credit he comes across to get to that 50,000 mark and buy one. That's may an adventure right there.
While "each GM should use their own discretion" sounds good, I've always found an actual guideline to be better. Regardless of weather or not you agree with the rules, having a yard (meter) stick is always useful.